Thermoelectric Composites Research Articles

Thermoelectric properties of GeTe-based composites prepared by spark plasma sintering containing Bi/Sb additive

Spark plasma sintering (SPS) is often used to prepare bulk thermoelectric (TE) materials, and the application of additives during the process may have a unique effect on constructing multiphase structures. The additives with low melting points can be often extruded or react with the matrix during the SPS process, leaving dense dislocations or multiphase structures to optimize the TE performance. In this paper, high-performance GeTe-based composites were prepared by SPS containing Bi or Sb additive. During the SPS process (sintering temperature ∼773 K), the Bi (melting point ∼544 K) or Sb (∼904 K) additive have the liquid-solid/solid-solid reaction with the GeTe matrix, respectively. The chemical reactions lead to the composite structures of the matrix and pseudo-binary alloy (GeTe)x(Bi2Te3) or (GeTe)y(Sb2Te3) with 7 ≤ (x, y) ≤ 9, resulting in a significant increase in the Seebeck coefficient by the energy filtering effect and the reduction of thermal conductivity by enhancing the phonons scattering. The influence of the Sb additive is better than that of Bi, therefore, the sample with 1 wt% Sb has a ZTmax value of 1.49 at 685 K, ∼157 % higher than that of the matrix (0.58). This study shows that SPS with appropriate additives is an effective method to prepare high-performance GeTe-based TE composites.

Modulating the Doping Effect for Carbon Nanotube-Based Thermoelectric Composites by Radical-Containing Naphthalene Diimides.

Carbon nanotube-based organic thermoelectric composites have garnered a significant amount of research interest due to their synergistic benefits of the high electrical conductivity of carbon nanotubes and the low thermal conductivity of organic materials. Nevertheless, the correlation between the organic molecular structures and the thermoelectric properties of these composite systems has remained largely unexplored. This study delves into the doping effects of radical-containing naphthalene diimides (NDIs) on single-walled carbon nanotubes (SWCNTs) through molecular engineering. It finds that the structures of radical groups crucially impact the frontier energy levels of NDI molecules, thus influencing doping effects on SWCNTs. Conjugated phenoxy radicals enhance p doping, yielding superior n-type thermoelectric properties, while nonconjugated radicals promote n doping, enhancing p-type characteristics. This work highlights the enormous potential of molecular engineering through modulating doping effects for novel TE materials in energy conversion and utilization applications.

Skin‐Like Soft Thermoelectric Composites with a “J‐Shaped” Stress–Strain Behavior for Self‐Powered Strain Sensing

AbstractPolymer thermoelectric (TE) materials present a promising alternative to actuating low‐power wearable electronics without an additional power source, among which poly(3,4‐ethylenedioxythiophene) (PEDOT):poly(styrenesulfonate) (PSS) is a promising candidate. However, it is too hard and brittle to integrate into wearables seamlessly and comfortably. Herein, PEDOT:PSS‐based TE composites with simultaneous softness and stretchability are fabricated by a water‐borne polyurethane (WPU) and PEDOT:PSS mixture containing the judiciously chosen ionic liquid (IL) and subsequent drop casting. The obtained composites show a stable TE voltage, high stretchability (>500%), ultra‐flexibility, and excellent sensitivity (gauge factor = 1251). More importantly, it exhibited “J‐shaped” stress–strain curves resembling human skins after a loading/releasing treatment. The skin‐like nonlinear elastic behavior combines enough softness in the “toe” region, high stretchability, and a strong strain hardening at the late deformation stage, enabling its seamless and comfortable integration with the human body. Given the desired mechanical performance and strain‐sensing capability, the composite is designed to serve as a self‐powered sensor with high sensitivity and accuracy, suggesting a high potential in human motion detection. This work demonstrates the versatility of the developed skin‐like PEDOT:PSS‐based composites in wearable electronics.

Constructing high-performance bulk thermoelectric composites by incorporating uniformly dispersed fullerene sub-nanoclusters

Sub-nanomaterials possess unprecedented size-dependent properties compared to conventional nanomaterials, which endow them with great potential in catalysis, biomedicine, sensors, and so on. However, their applications in thermoelectrics are unknown due to poor thermal stability and low yields. Herein, we construct a series of thermoelectric composites by incorporating highly thermally stable and commercial fullerene sub-nanoclusters (C60 or C70). We find that sub-nanoclusters as the second phase can conduce to optimized carrier concentration through charge transfer at interfaces while the carrier mobility is significantly enhanced due to atom orbital hybridization and size-dependent electrical scattering mechanism. Furthermore, the ultra-low thermal conductivity of C60 due to its distorted chemical bonding and sub-nanometer pore, and the interfacial thermal resistance greatly suppress the phonon transport. Consequently, the 0.15 mol.% C60/Bi0.4Sb1.6Te3 realizes an ultra-high ZT of ∼1.6 at 373 K, an excellent thermoelectric conversion efficiency of ∼7.4 %, and a huge cooling performance of ∼73 K. This work demonstrates the application of sub-nanomaterials in thermoelectrics and may shed light on other fields such as electronic devices, thermal management, and fullerene chemistry.

Thermoelectric cement-based composites containing carbon nanotubes (CNTs): Effects of water-to-cement ratio and CNT dosage

This study investigates the effects of the water-to-cement (w/c) ratio and carbon nanotube (CNT) dosage on the thermoelectric and mechanical properties of cement-based composites. Four different types of w/c ratio including 0.2, 0.3, 0.4, and 0.5 were employed with three different dosages of CNT, including 0.15, 0.3, and 0.5 vol% (≈0.39 wt%). Additionally, pristine CNT and nitrogen-doped CNT were adopted to fabricate p- and n-type thermoelectric cement-based composites, respectively. The Seebeck coefficient, electrical conductivity, compressive strength, and isothermal calorimetry were then measured and evaluated. The test results indicate that at a w/c ratio of 0.5, the use of CNTs enhances the compressive strength of cement paste by up to 65.9 %. The isothermal calorimetry measurement also confirmed the acceleration of the hydration reaction by the CNTs added to the cement paste. A w/c ratio of 0.3 with CNT dosages of 0.3 and 0.5 vol% was also found to be the most effective in improving the Seebeck coefficient. Additionally, the highest Seebeck coefficients for the p- and n-type thermoelectric cement-based composites are 743.0 and −428.3 µV/K, respectively. Considering that a CNT dosage of 0.3 vol% is most effective in enhancing the electrical conductivity, the best cement-based composites in terms of thermoelectric properties comprise a w/c ratio of 0.3 and a CNT dosage of 0.3 vol%.

In Situ Surface Polymerization of PANI/SWCNT Bilayer Film: Effective Composite for Improving Seebeck Coefficient and Power Factor

AbstractFlexible thermoelectric composites are promising in harvesting low‐temperature energies and have a wide range of applications in wearable devices. However, the uncontrollable interfaces between the components significantly scatter the transport of electrons leading to a reduced thermoelectric performance. In this research, hydrochloride acid doped polyaniline (PANI) is directly prepared on the surface of a single‐walled carbon nanotube (SWCNT) film to form a bilayer structure in controlling the interfacial filtering effect and preserve the high conductivity. Under optimal conditions, a PANI/SWCNT bilayer composite exhibits a Seebeck coefficient of 26 µVK−1 at near room temperature, and a high electrical conductivity of 1320 S cm−1 is maintained. The Seebeck coefficient is a twofold increase compared to a pure SWCNT film, while the power factor is three times the value from a pure SWCNT film and 500 times that from a solution‐mixed PANI/SWCNT composite. The enhanced thermoelectric performance is attributed to SWCNT‐assisted growth through π–π interaction promoting the formation of an oriented and compacted PANI layer, and the bilayer structure can also maximally maintain the electrical conductivity of the composite. The bilayer composite has then been investigated as an energy harvester and a temperature sensor, indicating its reliable performance in wearable applications.

Conductive Polymer-Based Thermoelectric Composites: Preparation, Properties, and Applications

Conductive Polymer-Based Thermoelectric Composites: Preparation, Properties, and Applications

Delocalized Charge Transport in Thermoelectric Composites of Semiconducting Carbon Nanotubes Wrapped with a P‐Type Polymer

Delocalized Charge Transport in Thermoelectric Composites of Semiconducting Carbon Nanotubes Wrapped with a P‐Type Polymer

Organic Thermoelectric Materials for Wearable Electronic Devices.

Wearable electronic devices have emerged as a pivotal technology in healthcare and artificial intelligence robots. Among the materials that are employed in wearable electronic devices, organic thermoelectric materials possess great application potential due to their advantages such as flexibility, easy processing ability, no working noise, being self-powered, applicable in a wide range of scenarios, etc. However, compared with classic conductive materials and inorganic thermoelectric materials, the research on organic thermoelectric materials is still insufficient. In order to improve our understanding of the potential of organic thermoelectric materials in wearable electronic devices, this paper reviews the types of organic thermoelectric materials and composites, their assembly strategies, and their potential applications in wearable electronic devices. This review aims to guide new researchers and offer strategic insights into wearable electronic device development.

Exceptional figure of merit achieved in boron-dispersed GeTe-based thermoelectric composites

GeTe is a promising p-type material with increasingly enhanced thermoelectric properties reported in recent years, demonstrating its superiority for mid-temperature applications. In this work, the thermoelectric performance of GeTe is improved by a facile composite approach. We find that incorporating a small amount of boron particles into the Bi-doped GeTe leads to significant enhancement in power factor and simultaneous reduction in thermal conductivity, through which the synergistic modulation of electrical and thermal transport properties is realized. The thermal mismatch between the boron particles and the matrix induces high-density dislocations that effectively scatter the mid-frequency phonons, accounting for a minimum lattice thermal conductivity of 0.43 Wm−1K−1 at 613 K. Furthermore, the presence of boron/GeTe interfaces modifies the interfacial potential barriers, resulting in increased Seebeck coefficient and hence enhanced power factor (25.4 μWcm−1K−2 at 300 K). Consequently, we obtain a maximum figure of merit Zmax of 4.0 × 10−3 K−1 at 613 K in the GeTe-based composites, which is the record-high value in GeTe-based thermoelectric materials and also superior to most of thermoelectric systems for mid-temperature applications. This work provides an effective way to further enhance the performance of GeTe-based thermoelectrics.

Significant Improvement in Thermoelectric Properties of Carbon-Reinforced Cementitious Composites Achieved by Regulating the Graphitization Degree of Expanded Graphite.

Building structures are exposed to direct sunlight for a long time, accumulating a large amount of low-grade thermal energy, which aggravates environmental pollution and energy consumption. Thermoelectric cement-based composites can realize the interconversion of thermal and electrical energy, showing great potential benefits in large-scale heat collection and energy conversion. Although a lot of exploration and research has been carried out on the thermoelectric properties of cement-based composites reinforced with carbon materials, the contribution of the characteristics of carbon materials, such as the graphitization degree, to the thermoelectric properties of cement-based composites is still unclear. In this article, the graphitization degree of expanded graphite (EG) was modulated by etching EG with an acid solution. The low graphitization degree improves the effective mass of carriers and aggravates the electron and phonon scattering at the interface of EG/cement-based composites. Low thermal conductivity was obtained while increasing the Seebeck coefficient of EG/cement-based composites. The power factor (17.1 μW m-1 K-2) and thermoelectric figure of merit (2.95 × 10-3) of the sample are increased by 18.6 times and 44.2 times, respectively, achieving the highest thermoelectric performance in cement-based composites reinforced with carbon materials. This study provides a direction for improving the thermoelectric properties of cement-based composites by structural regulation of carbon materials.

A strategy towards fabrication of thermoplastic-based composites with outstanding mechanical and thermoelectric performances

Compared to the great achievements in enhancing thermoelectric (TE) performance, little attention is paid to the mechanical (ME) performance of polymer composites although it is a prerequisite for practical applications. However, how to improve a trade-off between TE and ME performance is a great challenge, as the increase in ME performance is always along with the decrease in TE performance and vice versa. Herein, an enhanced trade-off is realized for ionic liquid (IL)-modulated flexible poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/ single-walled carbon nanotube (SWCNT)/polycarbonate (PC) composites. It shows a maximum power factor value of 8.5 ± 2.1 μW m−1 K−2 and a strong mechanical robustness is also achieved for the composite with a fracture strength of 43.4 ± 5.4 MPa and a tensile modulus of 3.8 ± 0.4 GPa. The TE and ME performances are superior to other thermoplastics-based TE composites, and even comparable to some conducting polymers and their composites. The high electrical conductivity of PEDOT:PSS/SWCNT and their strong interfacial interaction with PC are responsible for the enhanced trade-off between ME and TE performances. This work provides a new avenue to endow polymer composites with high TE and ME performances simultaneously and will promote their versatile TE applications.

Flexible Organic Thermoelectric Composites and Devices with Enhanced Performances through Fine-Tuning of Molecular Energy Levels

Flexible Organic Thermoelectric Composites and Devices with Enhanced Performances through Fine-Tuning of Molecular Energy Levels

Preparation of high‐performance bismuthene thermoelectric composites doped with graphene using UV‐curing 3D printing technology

AbstractThermoelectric materials are of tremendous interest due to their capacity to convert waste heat into useful electrical energy. Bismuthene nanosheets (BiNS) have recently emerged as a promising material for thermoelectric applications because of their unique two‐dimensional structure and low thermal conductivity. However, the development of efficient and scalable methods for preparing BiNS‐based thermoelectric composites with improved performance remains a challenge. In this study, we reported a novel approach to preparing a thin film BiNS‐based thermoelectric composite with UV‐curing 3D printing technology. The composite was prepared using BiNS as the filler and a UV‐sensitive resin as the matrix, which contained 5 vol% graphene as the third phase to enhance the thermoelectric performance of the composite. The results showed that the prepared composite exhibited stable thermoelectric performance, with a maximum power factor of 311.79 ± 13.90 μW/mK2 when the volume fraction of BiNS reached 95 vol%. The electrical conductivity of the composite improved significantly as the volume fraction of BiNS increased from 75 to 95 vol%. The Seebeck coefficient remained essentially unchanged in the range of 70.1–70.7 μV/K. These findings demonstrate the potential of BiNS‐based thermoelectric composites prepared by UV‐curing 3D printing technology for practical applications in energy harvesting and cooling devices.Highlights A simple, efficient and scalable method for BiNS thermoelectric composites. UV‐curing 3D printing technology was used for fabrication. The BiNS composite exhibited stable thermoelectric performance. The electrical conductivity of the composite improved significantly. This BiNS composite showed great potential in thermoelectric application.

High-Power-Density and Excellent-Flexibility Thermoelectric Generator Based on All-SWCNTs/PVP Composites.

Flexible polymer/single-wall carbon nanotube (SWCNT) composites are a vital component for wearable/portable electronics, but the development of their n-type counterpart is laggard. Furthermore, little attention is paid to the interaction between SWCNT and polymers, especially the unconjugated polymers, as well as the conversion mechanism of conduction characteristics. Here, the n-type flexible SWCNTs/Polyvinyl Pyrrolidone (PVP) films are successfully fabricated, where the oxygen atoms in PVP interacted with SWCNT via hydrogen bonds, which can lower the energy barrier of electron tunneling, providing the pathway for the electron transfer. Furthermore, with the increasing synthesis temperature, the hydrogen bonds strengthened and the thermal activation energy further improved, both of which enhanced the electron-donating ability of PVP, resulting in a high-power-factor value of 260µW m-1K-2. Based on the optimized SWCNTs/PVP films, a thermoelectric module is assembled, which achieved a power density of 400µWcm-2 at a temperature difference of 56K, coupled with excellentflexibility, showing a less than 1% variation of resistance after 5000 bending cycles. It shows the highest output-performance and the best flexibility among the reported SWCNT-based thermoelectric modules. This work provides significant insights into the interaction mechanism and performance optimization of hybrid thermoelectric composites, based on SWCNTs/unconjugated polymers.

Construct Amorphous Polymer Interface to Enhance the Thermoelectric Performance of Commercial Bi0.5Sb1.5Te3 Materials.

Interfaces, such as grain boundaries and phase boundaries in thermoelectric (TE) materials, play a crucial role in the carrier/phonon transport. Accurate control of the features of interfaces, including composition, crystalline nature, and thickness may give rise to a promising pathway to break the trade-off between phonon and carrier transport properties, which is essential to design high-performance TE materials. In this work, the amorphous polymer interface (API) layer is introduced to the p-type commercial Bi0.5Sb1.5Te3 (BST) TE material by the liquid-phase sintering process. Due to the larger mismatch in the acoustic impedance or phonon spectra between the amorphous polymer layer and the BST phase, the additional interfacial thermal resistance is introduced, which results in a large decrease in lattice thermal conductivity. It is found that the interfacial thermal resistance at the API is much higher than that of normal grain boundary and hetero interface reported in the literature. Conversely, taking advantage of the strong electron and phonon scattering, a large net get of ZT was achieved. A maximum ZT of ∼1.22 at 350 K was obtained in the BST/polyimide-0.5% sample, which is considerably greater than that of the commercial BST matrix (∼0.99 at 350 K). Furthermore, the optimized BST/polymer sample also exhibited almost 20% enhancement in hardness compared with the pure BST sample. This work has opened a new window for designing high-performance TE composites, which may extend to other material systems.

PREPARATION AND THERMOELECTRIC PROPERTIES OF Cux/Bi2Te3 COMPOSITES

Thermoelectric materials are the use of solid internal carriers to realize mutual conversion between thermal energy to electrical energy. Bismuth telluride (Bi<sub>2</sub>Te<sub>3</sub>) based thermoelectric materials are relatively mature near low temperatures. Due to the excellent conductivity of copper, the method of combining copper with a thermoelectric matrix to produce thermoelectric composite materials with better thermoelectric properties has always been a research focus in thermoelectric composite materials. Bismuth telluride thermoelectric composite materials (Cu<sub>x</sub>/Bi<sub>2</sub>Te<sub>3</sub>) samples with different mass percentages of purity (Cu) powder were prepared by spark plasma sintering (SPS) method. The influences of the Cu with different incorporation ratios on the performance of Cu<sub>x</sub>/Bi<sub>2</sub>Te<sub>3</sub> thermoelectric composites were investigated, such as differential thermal conductivity, thermal conductivity, Seeback coefficient, and ZT values. The Cu<sub>x</sub>/Bi<sub>2</sub>Te<sub>3</sub> thermoelectric composites with Cu chemical composition of 0.005 exhibited excellent Seebeck coefficients and ZT value at the heat source temperature of 325 K. The values are -289.46 μV/K and 1.69, respectively. The value of ZT is improved by about 2 times, compared with Bismuth telluride Bi<sub>2</sub>Te<sub>3</sub> thermoelectric materials.

A comprehensive investigation into thermoelectric properties of PEDOT:PSS/Bi0.5Sb1.5Te3 composites

Methodologies for enhancing the conversion efficiency of organic/inorganic thermoelectric composites, enabling future applications in self-powered wearable electronic devices.

High-performance thermoelectric composites via scalable and low-cost ink processing

This work demonstrates a remarkable room-temperature figure of merit zT of 1.3 for BiSbTe-based composites with excellent reproducibility using a scalable, low-cost ink processing technique.

Solution-based 3D printed thermoelectric composite for custom-shaped thermoelectric generator applications

To fulfill the ever-increasing power demands of deep space exploration, the output performance of radioisotope thermoelectric generators, the only accessible power source, must be enhanced in several aspects, including thermoelectric properties of materials, geometry, welding, etc. In this study, we present our results on the development of a novel thermoelectric slurry suitable for the 3D printing of thermoelectric generators with optimized leg geometry. The rheological properties of the slurry are optimized by combining proper amounts of organic solvent, binder, and bismuth telluride-based thermoelectric powder. The addition of Cu to the slurry as a conductive additive are also investigated. The homogeneous dispersion of Cu within the material not only increases the electrical conductivity of the final thermoelectric leg significantly but also promotes the crystallization of the thermoelectric particles during the sintering process. These effects result in 3D-printed thermoelectric composites exhibiting ZT values up to 0.91 and extended optimum operating temperature range. Thermoelectric modules composed of 3D-printed thermoelectric legs show excellent output performance and structural strength. This work could also produce other special shaped thermoelectric devices to match irregularly shaped heat sources to reduce contact heat loss and improve output performance.